1. Field of the Invention
This invention relates to an intensifying screen for radiographs, and more particularly to an intensifying screen for radiographs used in contact with a radiographic film in which the film is exposed to radiations of X-ray, .gamma.-ray or the like.
2. Description of the Prior Art
In radiographs, radiographic film is usually sandwiched between a pair of intensifying screens while the film is exposed to radiation. The radiations are visualized by the phosphor layer of the intensifying screens on both surfaces of the film so that images are recorded on the film with high optical density.
The intensifying screens are usually comprised of a support layer having a thickness of 0.2 to 0.5mm of a resinous sheet material such as polyester resin, polystyrene resin or the like or a support layer of synthetic paper, and a phosphor layer carried thereon. Further, some intensifying screens have a protective layer having a thickness of about 10.mu. applied on the phosphor layer.
As for the phosphor layer, fluorescent materials are usually employed and dispersed in a binder of high polymer such as nitrocellulose or polymethacrylic resins. As for the fluorescent materials, the following phosphors may be employed: self-activated calcium tungstate (CaWO.sub.4), silver activated zinc sulfide (ZnS:Ag), lead activated barium sulfate (BaSO.sub.4 :Pb), rare earth activated rare earth oxysulfide (e.g. Gd.sub.2 O.sub.2 S:Tb), rare earth activated rare earth oxyhalide (e.g. LaOBr:Tb), or the like. As for the protective layer, cellulose acetate, polymethylmethacrylate film or the like may be used.
When radiographic film is used, the film is put in a cassette together with a pair of intensifying screens which sandwich the film therein. In the cassette, the intensifying screens are in face contact with both surfaces of the radiographic film. When the film and the intensifying screens are taken out of the cassette, electrostatic discharge sometimes occurs between the film and the intensifying screens. The electrostatic discharge causes the film to be stained with static marks and lowers the image quality of the radiograph. When the support layer of the intensifying screens is made of a plastic material such as polyester or polystyrene, the electrostatic discharging is particularly apt to occur.
In order to prevent the electrostatic discharging, it has been known in the art, to mingle an antistatic agent in the phosphor layer of the intensifying screen, as disclosed in Japanese Patent Publication No. 3295/1966. It has also been known to provide an antistatic coating on the support layer of the intensifying screen before applying the phosphor layer thereon, as disclosed in Japanese Patent Publication No. 3296/1966.